共50条信息
在如图所示的多面体中,四边形\(ABCD\)是平行四边形,四边形\(BDEF\)是矩形.
\((2)\)若\(AD⊥DE\),\(AD=DE=1\),\(AB=2\),\(∠BAD=60^{\circ}\),求三棱锥\(F-AEC\)的体积.
已知四棱锥\(P-ABCD\),底面\(ABCD\)为菱形,\(PD=PB,\,\,H\)为\(PC\)上的点,过\(AH\)的平面分别交\(PB,\,PD\)于点\(M,\,N\),且\(BD/\!/\)平面\(AMHN\).
\((1)\)证明:\(MN\bot PC\);
\((2)\)当\(H\)为\(PC\)的中点,\(PA=PC=\sqrt{3}AB\),\(PA\)与平面\(ABCD\)所成的角为\(60{}^\circ \),求二面角\(P-AM-N\)的余弦值.
如图,过底面是矩形的四棱锥\(F-ABCD\)的顶点\(F\)作\(EF/\!/AB\),使\(AB=2EF\),且平面\(ABFE⊥\)平面\(ABCD\),若点\(G\)在\(CD\)上且满足\(DG=GC\).
\((1)\)求证;\(FG/\!/\)平面\(AED\);
\((2)\)求证:平面\(DAF⊥\)平面\(BAF\).
如图所示,在四棱锥\(P-ABCD\)中,\(AB⊥\)平面\(PAD\),\(AB/\!/CD\),\(PD=AD\),\(E\)是\(PB\)的中点,\(F\)是\(DC\)上的点,且\(DF= \dfrac{1}{2}AB\),\(PH\)为\(\triangle PAD\)中\(AD\)边上的高.
求证:\((1)PH⊥\)平面\(ABCD\);
\((2)EF⊥\)平面\(PAB\).
如图所示,在三棱柱\(ABC-A\)\(1\)\(B\)\(1\)\(C\)\(1\)中,\(E\),\(F\),\(G\),\(H\)分别是\(AB\),\(AC\),\(A\)\(1\)\(B\)\(1\),\(A\)\(1\)\(C\)\(1\)的中点,若\(D\)为\(BC_{1}\)的中点,求证:\(HD/\!/\)平面\(A_{1}B_{1}BA\).
如图,已知在直三棱柱\(ABC-{{A}_{1}}{{B}_{1}}{{C}_{1}}\)中,\(AB=A{{A}_{1}}=2\),\(\angle ACB=\dfrac{\mathrm{ }\!\!\pi\!\!{ }}{\mathrm{3}}\),点\(D\)是线段\(BC\)的中点.
\((1)\)求证:\({{A}_{1}}C\)\(/\!/\)平面\(A{{B}_{1}}D\);
\((2)\)当三棱柱\(ABC-{{A}_{1}}{{B}_{1}}{{C}_{1}}\)的体积最大时,求三棱锥\(C-A{{B}_{1}}D\)的体积.
如图所示,四棱锥\(P—ABCD\)的底面是矩形,\(PA⊥\)平面\(ABCD\),\(E\)、\(F\)分别是\(AB\)、\(PD\)的中点,又二面角\(P—CD—B\)为\(45^{\circ}\).
\((1)\)求证:\(AF/\!/\)平面\(PEC\);
\((2)\)求证:平面\(PEC⊥\)平面\(PCD\);
\((3)\)设\(AD=2\),\(CD=2\sqrt{2}\),求点\(A\)到平面\(PEC\)的距离.
如图,在长方体\(ABCD-A_{1}B_{1}C_{1}D_{1}\)中,\(AB=BC=EC=\dfrac{1}{2}AA_{1}\).
\((1)\) 求证:\(AC_{1}/\!/\)平面\(BDE;\)
\((2)\) 求证:\(A_{1}E⊥\)平面\(BDE\).
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